Without limiting the scope of the invention, its background is described in connection with bulk charge modulated device (BCMD) image sensors, as an example.
Heretofore, in this field, the bulk charge modulated device (BCMD) was developed to achieve optimal imaging performance in all aspects of image sensing. See Hynecek, J.,"Bulk Charge Modulated Transistor Threshold Image Sensor Elements and Method of Making", U.S. Pat. No. 4,901,129, issued Feb. 13, 1990, and Hynecek, J.,"BCMD--An Improved Structure for High-Density Image Sensors", IEEE Transactions on Electron Devices, Vol. 38, No. 5, May 1991, which are incorporated herein by reference. The BCMD possesses superior qualities such as zero reset noise, undetectable image smear and excellent blooming control. The sensor is fabricated using a modified p-well CMOS process, which makes it relatively inexpensive to manufacture and provides inherent advantages of low power and fast clocking speeds.
FIG. 1 is a schematic circuit diagram of a BCMD photosite array, indicated generally at 10. Each pixel in the image is represented by a photosite 12. The photosites 12 are arranged into an array of rows and columns, with the photosites 12 of each column connected to a common sense line 14, which is driven by a current source 16. After charge has been integrated in a row of photosites 12, a single row is selected by applying the appropriate signal to photosite gates 18 and the signal corresponding to the charge stored in the photosites 12 appears on sense lines 14 to be scanned.
A photosite reset operation is provided for by clearing the charge from the charge storage well of each photosite 12. This is accomplished by connecting the source and drain of the photosite 12 together and applying a negative potential to gate 18. An electronic shutter effect may therefore be implemented when using the array 10 which enables the device to be operated without the use of a mechanical iris to control the impinging light. This is accomplished by independently removing charge collected in a given row of photosites 12. By selecting the time interval between charge removal and readout, the effective integration time of the image sensor can be controlled to yield an optimal image.
However, with the increasing photosite density which is required to provide ever higher image sensor resolutions, the individual rows of the photosite array must be clocked at higher and higher speeds. Until now, the settling time of the photosite reset operation has been a limiting factor in the speed at which an image sensor array may be operated. It is therefore desirable to reduce the settling time of the photosite reset operation in order to increase the dynamic range of the BCMD image sensor. Accordingly, improvements which overcome this limitation are presently desirable.